1. Field of the Invention
The present invention relates to a distance measuring system for determining the distance to an object by means of a time-of-flight (TOF) method, in which light emitted from a light emitter is reflected by an object, reflected light that returns from the object is received, and an output corresponding to an amount of light received is used to measure the distance to the object.
2. Description of the Related Art
Heretofore, a distance measuring device that uses a time-of-flight (TOF) method is generally known as an ambient sensing method in which measurement of distances is realized in a contactless manner (see Japanese Laid-Open Patent Publication No. 2001-281336 and Japanese Laid-Open Patent Publication No. 08-313215). According to such a TOF method, light that is emitted toward an object from a light emitter, and a round trip time consumed after the light is emitted toward the object and until the light bounces off the object and returns, are sensed by a light receiver, whereby the distance to the object can be measured. For this reason, synchronization between a timing at which light is emitted from the light emitter and a timing at which light is received by the light receiver is regarded as a technical prerequisite.
However, the characteristics of the light emitting element per se that constitutes the light emitter, and the characteristics of the light receiving element per se that constitutes the light receiver, or in other words, the respective types of elements and wiring arrangements in relation to the light emitter and the light receiver, as well as circuit characteristics thereof, are influenced and changed by the surrounding environment including environmental conditions such as temperature and humidity. Thus, even if calibration is performed at the time of shipping, it is difficult to maintain measuring precision of the distance measuring system with respect to fluctuations in an ever-changing environment, leading to the risk that errors will increase. For this reason, there have already been proposed methods for carrying out synchronization between a light emitter and a light receiver using feedback controls (see International Publication No. WO 2008/047640 and Japanese Laid-Open Patent Publication No. 2009-236657).
According to the disclosures of International Publication No. WO 2008/047640 and Japanese Laid-Open Patent Publication No. 2009-236657, a difference (phase difference) in the phase of a light emission timing signal, which is output from a light emitting element drive circuit, and the phase of a light receiving timing signal, which is output from a light receiving element drive circuit, is detected in a phase comparison circuit, and by means of a delay time adjusting circuit disposed between the light emitting element drive circuit and a timing generating circuit for the light emitting element, the light emission timing, which is supplied to the light emitting element drive circuit, can be adjusted responsive to the aforementioned phase difference.